Dynamic Wetting of Aqueous Surfactant Solutions on Hydrophobic Solids and Water Subphases

Language:

English

Abstract:

Wetting is the ability of liquids to maintain contact with solids or other liquids. It is a commonly observed phenomenon in numerous natural and technological processes. Efficient wetting of liquids is crucial to painting, coating, printing, and drug delivery applications. Surface active agents (surfactants) are amphiphilic compounds which can lower the surface tension of liquid solvents. Adding surfactants to liquids is one common method to enhance wetting. Since the 1960s organomodified trisiloxane surfactants have been recognized as effective wetting agents for aqueous pesticide formulations because they fasten foliar uptake and wet larger leaf surface areas. Trisiloxane surfactants that possess the ability to promote rapid and extensive spreading of water on hydrophobic solids are known as superspreaders, and their wetting phenomenon is referred to as superspreading.

Numerous studies have been performed to reveal the peculiar properties of superspreaders and the underlying mechanisms of superspreading. The wetting area of superspreader solutions was found to increase linearly with time within the first several seconds. The highest wetting velocity was observed at a critical surfactant concentration. In the course of the years, the superspreading ability of trisiloxane surfactants was attributed to their molecular structure and to the way they aggregate in solutions. It was proposed that the driving force for superspreading is surface tension gradient over the spreading drop, which can be maintained for longer time by superspreaders than by non-superspreaders. Most of previous experiments were performed with video camera at low speeds (e.g., 500 frames per second or less). However, the investigation of early wetting with time scale of milliseconds is crucial to understand rapid adhesion phenomena. It also allows us to know when surfactants start to become effective in the wetting systems, which helps to understand the wetting mechanisms behind. The goal of this experimental thesis is to shed light on the early wetting stage of aqueous surfactant solutions on hydrophobic solids and water subphases. Different surfactants and solids are used for comparative investigations. The superspreading stage of surfactant solutions is systematically studied by changing the factors that are assumed to influence the surface tension gradient, such as surfactant concentration and relative humidity.

The experiments within this thesis are performed using high-speed video imaging with temporal resolution up to 0.02 milliseconds. The results show that the wetting processes of hydrophobic solids by surfactant-laden drops can be described by one, two, or three stages, depending on physicochemical properties of the surfactants and the solids used. Surfactants do not play a role in the early wetting stage, which is mainly dominated by inertia. After a characteristic time, inertial wetting goes over in viscous wetting. This stage has also a characteristic duration and is influenced by surfactants. Afterwards a superspreading stage is observed for superspreading drops only. The driving force in this stage is the surface tension gradient, which is influenced by the dynamic surface tension of the spreading drop. The superspreading dynamics depends strongly on the surfactant concentration, on the relative humidity, as well as on the substrate wettabilities. It is found that superspreader solutions only superspread on substrates whose wettability falls within a narrow range. Conversely, on water subphases superspreader and non-superspreader solutions behave similarly.

The work in this Ph. D. project completes one gap - early wetting dynamics of surfactant solutions – in prior work. The findings reveal different wetting stages with different characteristic duration. The action times of surfactants during the wetting process have been assessed. Moreover, this study provides more evidence for the surface tension gradient as a driving force in the superspreading stage. Therefore, the findings represent a significant step forward for surfactant-enhanced wetting and superspreading. They can also offer guidance on practical applications, e.g., crop spraying. By using superspreaders under proper conditions, the wetting performance can be maximized and a cost reduction can be achieved.